In recent years, with the growing concern over environmental problems, the demand for good fuel economy for automobiles has been becoming stronger, and there is also a demand for excellent fuel economy for rubber compositions used for automobile tires. As rubber compositions for automobile tires, a rubber composition containing a conjugated diene polymer such as polybutadiene or butadiene-styrene copolymer and a reinforcing agent is used.
As polymers for enhancing fuel economy, and polymer compositions having good fuel economy, for example, JP-A-7-102117 (JP-A denotes a Japanese unexamined patent application publication) proposes a polymer obtained by the radical polymerization of butadiene, styrene and a vinyl pyridine using an organic peroxide as an initiator, and a polymer composition containing the polymer and carbon black. JP-A-63-186748 proposes a polymer obtained by the living anionic polymerization of butadiene and styrene using an organic peroxide as an initiator, and then the modification of one end of the obtained polymer with a silane compound containing a dialkylamino group and an alkoxy group, and a polymer composition containing the polymer and carbon black.